When the discovery of Giganotosaurus was announced to the world in 1995, the news caused a sensation. For almost all of the 20th century, paleontologists had believed Tyrannosaurus to have been the largest of all theropods. However, Giganotosaurus, from the late Cretaceous of southern Argentina, proved to be at least as large. In their description of Giganotosaurus, Argentinian paleontologists Rodolfo Coria and Leonardo Salgado reported a length of 41 feet (12.5 m) - longer than some Tyrannosaurus skeletons.

The first Giganotosaurus discovery consisted of an incomplete skeleton. Paleontologists could tell that this new dinosaur was an allosauroid - it was obviously closely related to animals like Allosaurus and, especially, Carcharodontosaurus - but because the material was so incomplete, the estimates of its size were far from conclusive. Since 1995, a number of new discoveries have been made. In one case, several Giganotosaurus skeletons were found close together. This suggests that these animals may have moved around in groups.

In early 2000, a team of Argentinian and Canadian paleontologists announced the discovery of a well-preserved, slightly younger relative of Giganotosaurus in Argentina. The newly discovered animal closely resembled Giganotosaurus but was larger - it may have been as long as 46 feet (14 m). Early press reports suggested that it was longer than the largest known Tyrannosaurus.

Giganotosaurus lived alongside a number of giant sauropods. Some of these had bony plates on their backs, which may have afforded them some protection against attack from above. Only one theropod in the region was large enough to attack them from above - that predator was Giganotosaurus.